Fix failure with PO translation linter when there are no translations

When the HTML todolist contained a string that had no translations this
method was producing an error as it tried to call `.include?` on a `nil`
object.

Fix this by checking for the presence of translations first.

Caused by: https://gitlab.com/gitlab-org/gitlab/-/issues/217933
Errors appearing in:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38772

lib/gitlab/i18n/translation_entry.rb

@@ -106,7 +106,7 @@ module Gitlab
       end
 
       def translations_html_allowed?
-        html_allowed.present? && all_translations.all? do |translation|
+        msgid_html_allowed? && html_allowed['translations'].present? && all_translations.all? do |translation|
           html_allowed['translations'].include?(translation)
         end
       end

spec/lib/gitlab/i18n/translation_entry_spec.rb

@@ -328,5 +328,13 @@ RSpec.describe Gitlab::I18n::TranslationEntry do
         expect(entry.translations_html_allowed?).to be false
       end
     end
+
+    context 'when the todolist only has the msgid' do
+      let(:html_todo) { { 'plural_id' => nil, 'translations' => nil } }
+
+      it 'returns false' do
+        expect(entry.translations_html_allowed?).to be false
+      end
+    end
   end
 end